In the following description the term “turbine” is used to refer to rotary engines having a rotating part and a stator part force coupled by a fluid medium such as water or gas. Of particular interest for the present invention are axial turbines comprising radially arranged fixed stator blades or vanes alternating with radially arrangements of moving rotor blades. Movements are generally defined as movements relative to a casing or housing.
A common problem encountered in the design and operation of turbines is the leakage between the tips of the rotor blades or any circumferential shroud attached to them and the housing. The operation of a radial turbine requires a minimum of tip clearance between the rotating blades and the stationary wall of the casing or any extensions thereof. The gap, however, gives rise to a leakage flow, which is driven by the pressure difference between the pressure side and the suction side.
To reduce tip leakage the gap between the rotating parts and the static parts by appropriate seals may be closed. The most common type of seal used for this purpose is the labyrinth seal. A labyrinth seal has typically a number of radially extending annular knives on one part and a corresponding annular seal land on the other part or an arrangement of threads or grooves. All types of labyrinth seals have the common feature of providing a tortuous path for the fluid through the gap. For the purpose of preventing tip leakage in a turbine, the seal often takes the shape of a complete ring usually assembled as halves or segments within and supported by the casing.
As labyrinth seals are used in designs of turbine manufacturers, it suffices for the purpose of the present invention to emphasize that such seals are complex shapes, which have to be machined to exacting tolerances in order to function properly. Any movement of the parts of the seal from their default positions during operation generates usually a significant increase in leakage or friction between the moving and the static part. Known labyrinth seals for rotating blades have been integrated into the inner casing of a turbine as well as into diaphragm structures or ring-shaped carriers.
To accommodate relative movement of the parts of the seal in case of a radial expansion or shrinkage of the rotating parts, some seals are assembled as spring-backed packages. In a spring-backed seal, the elastic force pushes one part of the seal against the other and thus avoids widening gaps or excessive friction. For example, the AEG document Title: “Spezielle Konstruktionsaufgaben aus dem AEG-Grossturbinenbau” by Hans Reuschke (DK 621.165-181.2: 62.0022) describes in the section “Wellendichtung”, pp. 90-91, spring-backed rings supporting a seal between diaphragms.
Alternatives to the labyrinth seals are brush seals and finger seals. These types of seals include generally a plurality of flexible members mounted on one part and sealingly engaging a suitable surface on the other part.
A further alternative, which is however less commonly applied, is the film riding seal with two suitable shaped engaging surfaces. As the turbine rotates a thin film of fluid is generated between the surfaces with a small lifting force to keep them apart. Typically an elastic element is included in the seal design to exert a restoring force, which reduces friction during start-ups and counters the lifting force and maintains an approximately constant gap between the sealing surfaces.
In the design of a turbine, particularly in case of retrofitting an existing old turbine with modern and more efficient subparts such as blades, the choice of how to mount seals is often limited. In some retrofits, it may not be possible or even desirable to mount the static part of the seal directly onto or otherwise rigidly connected to the casing of the turbine. The design described in the co-owned published United States patent publication no. 2008/0170939, incorporated by reference herein in its totality, may serve as an example to illustrate seal designs where the static part of the seal is not rigidly connected to the casing.